1. Field of the Invention
The present invention relates to a motor driving device and a control method of the motor driving device, and more particularly, to a motor driving device configured to control driving of a motor based on command information of a target number of rotations and detection information of a number of rotations of the motor, and a control method of the motor driving device.
2. Description of the Related Art
As a method of controlling a rotating speed of a motor (for example, a brushless DC motor used as a fan motor or an electric fan motor) by a motor driving device, there has been used a control method in which a command signal is input from an outside, and a rotating speed of the motor is controlled according to the command signal. For example, as the command signal, a clock signal may be used. When the clock signal is used as the command signal, the clock signal and a signal indicating the number of rotations of the motor are compared to control a speed command value of the motor such that the clock signal and the signal indicating the number of rotations become same. Thereby, the rotating speed of the motor is controlled.
In this motor driving device, when the clock signal is stopped, the motor is stopped by a free run stop method in which all phases are turned off in response to the stop of the clock signal. However, according to this stop method, regenerative current flows while the motor is being rotated by its inertia, so that a noise (abnormal noise) is generated.
FIG. 6 shows an example of a control operation of a related-art motor driving device.
FIG. 6 shows changes of a clock signal Sc, a signal Sr indicating the number of rotations of a motor and an operating mode of the motor driving device when the motor is stopped by the free run stop method. Here, a case is exemplified in which the clock signal Sc is changed to 500 Hz, 1000 Hz and stop (0 Hz) over time.
That is, while the motor is being driven (driving mode) at a driving state where the signal Sr indicating the number of rotations is 500 Hz, when the clock signal Sc is changed from 500 Hz to 1000 Hz at a time t11, the control of accelerating the motor is performed, so that the signal Sr indicating the number of rotations is changed to 1000 Hz. When the clock signal Sc is stopped at a time t12, the operating mode is shifted to a deceleration mode and the motor starts to decelerate. At a time t13, switch elements corresponding to all phases of a motor driving unit become off, the operating mode is shifted to a stop mode and the motor is decelerated by a stop method of inertia rotation (the free run stop method). At a time t14, the rotation of the motor is stopped.
Here, in this free run stop method, the regenerative current flows from the time t13 to the time t14 (a time period of the inertia rotation), and an abnormal noise (for example, a clicking noise) may be generated.
In relation to this problem. JP-A-H7-184399 discloses a motor driving device configured to prevent generation of a noise due to a rapid stop of rotation. The motor driving device detects an operation stop based on a drop of a supply voltage to a driving circuit when an operating switch is turned off, and forcibly turns off all switch elements of the driving circuit based on the detection.
However, in the motor driving device of JP-A-H7-184399, a following problem occurs. That is, in this device, in order to suppress generation of noises, the voltage drop is detected when the power supply is turned off, so that the operation stop is detected. Therefore, when the power supply is on, even though the operation stop (the driving stop: stop of the clock signal) is detected, the device cannot cope with this situation. Also, this motor driving device is not configured to detect a signal indicating an actual driving state of the motor to thus detect the operation stop, but is configured to detect the operation stop based on the voltage drop, which is an indirect stop condition. Therefore, the accuracy of detecting the operation stop is low. For example, even when the voltage drop is detected and the operation stop is thus detected, there is a possibility that the motor is being rotated by its inertia.